DNS Notify

DNS Notify
is a revision to the DNS standard (RFC 1996) that proposes that the master server for a zone notify certain secondary servers in that zone of changes, and the secondary servers can then check to see whether they need to initiate a zone transfer. This process can help improve consistency of zone data among secondary servers.

To determine which secondary servers in a zone to send the changes to, the master server for the zone contains a
notify list
, which is a list of the IP addresses for those secondary servers. The master server for the zone notifies only listed servers when zone updates occur.

When the local zone on a master server for the zone is updated, the following events take place:

The
SerialNumber
field in the SOA record is updated to indicate that a new version of the zone has been written to a disk.

The master server then sends a notify message to other servers that are part of its notify list.

All secondary servers for the zone that receive the notify message respond by initiating an SOA-type query back to the notifying master server to determine if the zone of the notifying server is a later version than the currently stored copy of the zone.

If a notified server determines that the serial number used in the SOA record of the zone of the notifying server is higher (more recent) than the serial number used in the SOA record for its current zone copy, the notified server requests an AXFR or IXFR zone transfer.